Wednesday, June 30, 2010

Oh noes we're all going die...by 2200

I'm surprised that I'm the first to get to this, as I was trying to ignore it yesterday in the hope that someone else would blog it. Maybe no-one else reads the Independent, but the Times went behind a paywall recently so I've been shopping around.

"Almost all" means 9 out of 14, who put this probability at 90% or greater, assuming a high emissions trajectory getting to about 1000ppm CO2 over the next 200 years. On reading the paper (here), the definition of "tipping point" seems conveniently vague. Originally a precisely-defined concept relating to hysteresis and bifurcation, it was devalued beyond all useful meaning in the Lenton et al paper, which define it as any point at which a small forcing change results in a "qualitatively different state". Without any clear definition of what "qualitatively different" means this seems more of a political construct than a scientific one.

I thought that this quote in the Indy was quite remarkable:

“We are certainly capable of committing ourselves to an emissions trajectory that make 1,000 ppm in 2200 almost inevitable if we make the wrong decisions over the next 20 years,” Dr Allen said.

I can't help thinking how incredibly fortunate we are to have worked this out just in time. Just imagine if Dr Allen had calculated that it was actually the last 20 years that were critical, and we were already committed to this dire long-term future. I suppose we would just have to party like it's 2199. Less facetiously and more directly, it seems an astonishing level of hubris that anyone (and a mere climate scientist at that) could claim to know how the next 200 years of socioeconomic development will be irrevocably (and predictably) affected by decisions we take in the next couple of decades. It would make about as much sense to claim that if only Spencer Perseval had thought more carefully he could have diverted us from our path to 393ppm today.

Despite the newspaper headline, the paper isn't really about tipping points. It's a much more wide-ranging survey of a handful of "experts who represent a range of main-stream opinion" (in the words of the paper) which follows up on a similar survey back in 1995 (Morgan and Henrion). Given the small sample of 14, I was surprised to see that no fewer than 3 (ie more than 20% of the total) were co-authors on the Stainforth et al CPDN paper which was hyped beyond all reasonable bounds (eg see here and here). And several of the rest are responsible for the silly "observationally constrained pdfs" for climate sensitivity, which as we pointed out here and here are simply pathological by construction.

Anyway, here are their new climate sensitivity estimates, presented as box and whisker plots. There is no explanation of exactly what the bars and lines mean, but the box is 25-75% and the dot is the median:The bracketed values under each plot are the probabilities for sensitivity greater than 4.5C, so we see that 4 people put this value at 30% or greater. This seems remarkable when not a single GCM from the AR4 had such a high value, and all the decent quantitative analyses point more or less strongly to a rather lower value. Number 4 seems sane, but how anyone can claim to be certain that the sensitivity is not as low as 2.3C (number 10) seems absurd to me. Number 2 obviously has a misprint somewhere as the 25% number is incompatible with the box plot. Numbers 2, 4, 6 and 8 have extra grey plots which refer to their contributions to the previous work of Morgan and Henrion which they also participated in:

(Rotated for the sake of matching the shape.) The 4 repeat participants are Karl, Schneider, Stone and Wigley, though not necessarily in that order.

So even though as far as I can tell everyone accepts that the fundamental points we make in our two papers are valid, they still stick to these old discredited results with long tails to high values. - in fact the answers are more alarmist than 15 years ago. Makes us wonder why we bother...

DC, the caption doesn't suggest that - and #2 is so silly (best estimate greater than 4!) that I expect the plot is wrong.

MC, unfortunately however much one tortures the data it will not confess to such a low value. If sensitivity was so small, we'd never have had ice ages and the planet would have barely warmed over the last century. A shame as it would solve a lot of problems (though not ocean acidification).

Well talking of AR5, the two CLAs plus two more authors on the most relevant chapter (long term climate change) are in this set of 14 - and none of them are the sane #4...(ok I accept several of the other pdfs are not really too ridiculous either).

Incidentally one participant in this new work is the person I think I mentioned some time ago who openly advocated exaggerating in opinion polls such as this in order to encourage "action".

I could hardly have expected to be included in a group of only 14, and I'd have pretty much agreed with 4, so it's not that my views were not represented. But I'd like to hear what several people use as justification for their responses. If they think our (elementary) arguments are invalid, then it's disappointing that they haven't tried to explain why.

Sorry, I just don't see how some people paying attention to long and/or fat tails somehow invalidates what you've done over the years? It just seems to me (still) that we are doing unprecedented forcings that have no analogue in paleoclimate data (hell I'd even extend that to how we extract oil and leave it gushing under a mile of ocean etc).

The "fun" stuff is looking at the worst-case scenarios, right? I mean if everyone just said "2.9C+/-.8C" what would there be to do? ;-)

It's the very high probabilities for high sensitivity that I object to - there really is no evidence at all for this, and lots against, once you realise that the widespread praxis of "take a uniform prior and ignore almost all the data" is pathological and guarantees a long fat tail irrespective of what the observations are.

[It's not as if there isn't a mountain of evidence positively pointing to ~3C either.]

Oh that removed comment was more interesting :-) I was going to respond that I think that the tipping points stuff is potentially/originally interesting but it's been turned into a pretty meaningless catch-phrase that is more of a device for getting headlines than actually advancing our understanding of the climate system.